Most modern submarines constructed in the west utilize a single Pressure Hull configuration with the Main Ballast Tanks (MBT) situated at the fore and aft ends of the submarine. Typically, the reserve of buoyancy (ROB) is in the order of 11% of the surfaced displacement of the boat. A better distribution of weights is achieved by incorporating some of the MBT capacity in a midship location which results in better balance and handling, particularly when the submarine is surfaced. Some older designs (e.g. Skipjack and Permit classes) had such a configuration. However, the incorporation of these tanks was the result of giving the Pressure Hull (PH) a complicated and less than ideal shape in order to withstand deep diving pressure. Where the midship MBTs were located, the PH was narrowed or waisted, thus allowing the resulting space between the PH and the outer hull to be used for MBTs. This narrowing of the hull was accomplished by welding circular conical PH sections to the cylindrical PH sections. This gave rise to undesirable stress areas where the conical sections joined the cylindrical sections and which had to be met with heavy scantlings and bulkheads. In addition, the safety margin offered by a ROB of only 11% is very small. Should an incident take place at depth that produces a breach of the pressure hull and renders inoperative some MBTs, a subsequent emergency blow might be insufficient to establish positive buoyancy and the boat may sink.
Increasing the designed ROB of a submarine and distributing the MBTs over three main areas rather than only two, should reduce the amount of reserve buoyancy that would be lost in such an incident and thus improve the chances of saving the submarine.
Torpedo tubes are usually limited to 4 and are situated in the fore end of the submarine with a complicated system of tanks used to fire the torpedoes and compensate the weight of these with sea water. The torpedo room is located behind these tubes. Space considerations limit the capacity of most American SSNs to about 22 weapons. With the advent of submarine-launched air cruise missiles such as Tomahawk and Harpoon, this capacity was insufficient to ensure an adequate mix of weapons and guarantee the submarine a sufficient minimum number of each type of weapon to meet many mission requirements. In addition, due to the nature of the tactics involved in the use of air cruise missiles (particularly against heavily defended surface ships), there was a need to be able to fire more of these weapons quickly. The 688I class of attack submarines solved this problem by incorporating a Vertical Launch System (VLS) consisting of 12 tubes mounted vertically in the forward MBT area and dedicated exclusively to carrying air cruise missiles. Each tube carries one round and can only be reloaded when the submarine is docked. The new Seawolf class SSNs solves the problem by having 8 torpedo tubes and a capacity of about 48 weapons with the added advantage that these are general purpose tubes which can fire a full range of attack submarine weapons, thus permitting greater flexibility in configuring the weapons mix.
Unfortunately, this increase in weapons-carrying capacity is one of the reasons for the tremendous increase in the size and cost of attack submarines: whereas a Sturgeon class boat displaces some 4700 tons submerged, a 688 displaces 6900 tons and the Seawolf around 9100 tons.
Reloading torpedoes is a rather long process which, in the case of a 688 class submarine, involves dismounting part of the interior floor space to assemble a ramp mechanism on the deck so that weapons can be lowered on a slide to the torpedo room and placed on their respective racks. The entire process of reloading a full weapons load is reputed to take some 12 hours.
Firing a weapon from a torpedo tube also takes rather longer than is desirable. First the weapon must be loaded into the tube and the electrical signal connections made. The breech is closed and water from the Water Round Torpedo (WRT) tank is used to fill the space between the torpedo and the tube. The torpedo is "tested" by the fire control team to ensure it is in working order and the relevant targeting instructions are transmitted to the guidance system. Pressure is equalized with surrounding sea water by opening a slide valve and, finally, the pressure cap and exterior doors are opened and the torpedo can be fired. Once fired, the tube remains filled with water which partly compensates for the weight of the weapon. However, as the weapon is usually heavier than the water it displaces, in order to maintain trim, the Automatic Inboard Venting tank must take on sufficient water to compensate for the difference. If the weapon fired is a wire-guided torpedo, the tube cannot be reloaded unless the decision to cut the wire is taken. Reloading a torpedo tube takes even longer. The muzzle cap and slide valves must be closed and the water from the tube drained into yet another tank called the Torpedo operating Tank situated where it can continue to maintain longitudinal balance and with sufficient capacity to take on all the water required to compensate for the loss of weight which would result if a full load of weapons were discharged. The breech door can now be opened and the tube inspected and cleared of any remaining wires and dispensers before the crew can proceed to reload a new weapon.
There are a number of additional disadvantages to forward-mounted torpedo tube arrangements:
The process of preparing a tube for firing and then actually firing the weapon results in a large amount of noise being generated in the vicinity of the bow sonar which is the main sonar array. This noise causes the sonar to be temporarily "blinded". Furthermore, firing a torpedo from the bow implies having to give it sufficient impulse to overcome the forward speed of the submarine and to ensure that the submarine will not hit the torpedo should its engine fail to start.
Wire-guided torpedoes require that the doors of the torpedo tubes remain open until the wire runs out or the decision is taken to cut it. Since this wire can be over 10 miles long, the submarine may be manoeuvring for a long while with open doors near the bow area which cause a certain amount of turbulent noise. There is also a risk that the wire may rub over the bow sonar casing thus causing more interference.
The acute angles the guidance wire may take with respect to the tube muzzle or the hull door opening may cause the wire to break, particularly during evasive manoeuvres. To reduce this possibility, some torpedoes have a wire dispenser which is attached to the breech end of the tube in addition to a dispenser at the stern of the torpedo. The wire is paid out from both ends in order to reduce its tension. Additional protection for the wire is usually provided by a reinforced "flexhose" which extends through the tube and outside the hull and through which the guidance wire runs. However, there is a risk that this flexhose could later interfere with the closing of the tube pressure cap or hull door.
It can thus be concluded that the traditional location of the torpedo tubes in the vicinity of the bow is undesirable and, furthermore, with the widespread use of guided weapons that do not need to be aimed, unnecessary.
1. Should a major mishap occur in a submerged submarine and it be unable to surface, there is no adequate method of evacuating the crew. Current practice in virtually all submarines (except for the German IKL type 1500 submarines which are equipped with an escape "pod" which can carry the entire crew to the surface) involves the use of "free ascent". Most submarines have at least one escape trunk which can fit 2 or 3 men at a time. The trunk is flooded, pressure equalized with the sea, and the hatch is opened. The crew members in the trunk then float to the surface. The hatch is closed, the trunk is emptied into a tank inside the PH and another 2 or 3 crew members climb inside and the process is repeated. This system can only be used at shallow depths. In addition, surface weather conditions must be taken into consideration. Considering that a 688 class boat carries a crew of about 130 men and is equipped with only two of these trunks, the process of abandoning the submarine in these circumstances is clearly insufficient. The only other rescue method is by the use of specialized rescue submarines (such as the DSRV) of which the U.S.N. has only 2. In case of distress, an emergency buoy is floated to the surface to communicate the location of the submarine. The DSRV is transported by ship or submarine to the vicinity of the distressed submarine where it locates it and attaches itself to one of the escape hatches. Up to 24 crew members can board the DSRV at one time. The DSRV then returns to the assisting ship where it deposits the crew, and returns for another load of men. The main problem with this system is the time necessary to transport the DSRV to the site, locate the submarine and evacuate the entire crew. In addition, rescue can be complicated should the distressed submarine be laying on its side, at an extreme angle or in hostile waters. If the submarine is sinking in deep waters, the crush depth of the hull may be surpassed well before the DSRV can arrive. Thus, the DSRV will only serve for the rescue of the crew whose submarine has bottomed out at less than crush depth.
2. Because the trunk escape system is also the principal method of delivering divers while the submarine is submerged, most submarines are limited to delivering and recovering 2 or 3 divers per trunk at any one time. These trunks can also be fitted to act as decompression chambers. Should the divers require decompression for a length of time before entering the interior atmosphere of the submarine, then the capacity of the trunks may determine the maximum number of divers who may be recovered from an operation. Since there are occasional requirements for submarines to deliver fairly large numbers of divers during covert operations, the process of delivering and recovering these teams requires the submarine to be close to the surface for a considerable length of time, thus increasing the risk of detection. This has led some navies to build a few submarines specially adapted for this sort of mission. Unfortunately, these submarines have necessarily traded off some of their more conventional capabilities in order to meet these requirements.
3. The fairwater sail or bridge fin is a totally undesirable appendage when viewed from any hydrodynamic or hydrostatic aspect or, indeed, from any other aspect including stealth. It causes considerable drag high above the centreline axis which causes a bow-up pitching moment which, in turn, overrides the other hydrodynamic effects on the hull and so determines the settings required on the forward and after hydroplanes to allow the submarine to maintain a straight and level path. When the submarine is heeled into a turn, the bridge fin causes lift which can result in a "snap-roll". For this reason, some Navies have adopted separate two-man control for planes and helm thus increasing crew size. At speed, this fin can generate vortices which produce noise. When surfaced, it is the single most visible and characteristic appendage of a submarine--typically offering over 350 square feet of visible area well above the waterline which announces "submarine" to any observer. It also adds considerable weight topside which is the worst possible place. The hull of a 688 class submarine is about 33 feet in diameter but, because of the bridge fin, draws some 50 feet of water submerged. Furthermore, at periscope depth, the top of the bridge fin is only a few feet under water where wave motion can affect the stability of the submarine and where there is a risk of collision--particularly in crowded coastal waters. The only reason this appendage exists is because it is a convenient place to locate periscopes, antennas, snorkels, and a surface piloting or bridge position. In addition, the location of the control room underneath the bridge fin is dictated by the need to have the periscopes available there. The amount of space required by the periscopes also makes the control room much larger than would otherwise be necessary.
4. Long-range low-frequency sonars are housed in flank arrays. These arrays function best when mounted on constant-diameter sections of the hull, separated as far as possible along the length of the submarine and when recessed into a smooth hull so that flow turbulence is kept to a minimum. However, because of the location of the torpedo tubes, associated tanks and other equipment which crowd the forward MBT area, and the lack of midship MBTs, U.S.N. submarines must mount their flank arrays outside the pressure hull. This results in a bulge on each side of the hull where the arrays are fitted which adds drag and causes flow disturbance around the bulges. The turbulent noise generated causes interference and degrades the effectiveness of these sonars, thereby limiting their use to a short speed range. Furthermore, because the bulges which house these arrays are rather fragile, they are often mounted below the centreline of the hull where they do not protrude beyond the maximum beam of the PH so as to reduce the probability of damage when docking. Placing these arrays in this manner somewhat limits their ability to "listen" for sounds coming from other directions.
5. In many modern attack submarines the lack of habitable space for the crew gives rise to the practice of "hot-bunking". This constriction is related to the small ROB of these submarines. Hot-bunking in a 688I class submarine is reputed to affect about 40% of the junior enlisted personnel. It forces a rigidly set schedule for many berthing spaces which, in turn, tends to dominate the schedules of these crew members. It leads to lack of sleep and to an inflexibility in schedules which, ultimately, must affect the efficiency of the crew. The lack of sufficient berthing and the need of a certain minimum of privacy make submarines the only ships in the U.S. Navy that cannot have women crew members. Naturally, this implies that the Navy must forego fully 50% of all the motivated, intelligent and qualified young citizens who might aspire to crew submarines.
6. In recent years, there has been an increased demand for submarine missions that take place in littoral or shallow waters where the risk of detection is greater. A submerged submarine can leave a wake that is detectable from the air. The size of this wake increases with greater displacement and speed and less depth. The probability of detection by active sonars and magnetic anomaly detection equipment also increases with greater displacement and less depth. In addition, a submerged submarines' ultimate limitation in navigating shallow waters is its total submerged draught. Among the possible solutions to this dilemma, there should be a requirement for submarines to be smaller and stealthier while not losing any of their "blue-water" capabilities.
It is an object of the present invention to reduce at least some of the above mentioned problems.